| Because of diffraction, propagating radiation cannot be localized to dimensions smaller than half the optical wavelength and hinders a comprehensive analysis of the structural, vibrational and electronic properties of materials with size dimensions below one hundred nanometers. To overcome this limit, a nanoscale optical antenna is used to localize radiation to length-scales much smaller than the wavelength of light. By placing a laser-irradiated optical antenna, such as a bare gold tip, a few nanometers above a sample surface, (i.e. into the samples near-field), an optical interaction between the confined field and sample is used to induce a spectroscopic response within an interaction volume of (20 nm)3. My doctoral research has revealed how both phonons and excitons are confined in low-dimensional structures. By using near-field optical spectroscopy vibrational modes characteristic of individual single-walled carbon nanotubes (SWNTs) have been mapped with ultra-high spatial resolution. My work has shown that vibrational modes in SWNTs can be localized to regions of 20 run. In addition, it is shown that the out-of-plane vibrational modes experience greater Raman enhancement compared to in-plane vibrational modes in SWNTs. By combining near-field Raman and near-field photoluminescence measurements it is have shown that excitons are localized to defect-rich regions in semiconducting SWNTs, revealing that bright optical emission in SWNTs originates from bound excitons. Furthermore, it is demonstrated that the quantum efficiency in SWNTs can be increased by over two orders of magnitude using laser-irradiated gold tips. Finally, near-field Raman microscopy was applied to investigate structures buried beneath a capping layer with nanoscale (30 nm) resolution. |
